Surgery on the human eye has become commonplace in recent years. Many patients pursue eye surgery as an elective procedure, such as to avoid the use of contacts or glasses, and other patients may find it necessary to pursue surgery to correct an adverse condition in the eye. Such adverse conditions may include, for example, cataracts or presbyopia, as well as other conditions known to those skilled in the art that may negatively affect elements of the eye.
The anatomy and physiology of the human eye is well understood. Generally speaking, the structure of the human eye includes an outer portion, also referred to as a layer, formed of two parts, namely the cornea and the sclera. The middle layer of the eye includes the iris, the choroid, and the ciliary body. The inner layer of the eye includes the retina. The eye also includes, physically associated with the middle layer, a crystalline lens that is contained within an elastic capsule, referred to herein as the lens capsule, or capsular bag.
Image formation in the eye occurs by entry of image-forming light to the eye through the cornea, and refraction by the cornea and the crystalline lens to focus the image-forming light on the retina. The retina provides the light sensitive tissue of the eye.
Functionally, the cornea has a greater, and generally constant, optical power in comparison to the crystalline lens. The power of the crystalline lens, while smaller than that of the cornea, may be changed when the eye needs to focus at different distances. This change, or “accommodation,” is achieved by changing the shape of the crystalline lens. Accommodation, as used herein, includes the making of a change in the focus of the eye for different distances. For example, in order to change the shape of the crystalline lens for accommodation, the ciliary muscles may contract to cause ligaments that support the crystalline lens to relax, thereby allowing the crystalline lens to become more rounded.
The iris operates to change the aperture size of the eye. More specifically, the diameter of the incoming light beam is controlled by the iris, which provides the aperture of the eye, and the ciliary muscles may contract, as referenced above, to provide accommodation in conjunction with any needed change in the size of the aperture provided by the iris. The opening, or aperture, in the iris is called the pupil.
Correction of defects or degradation in the aspects of the eye may occur surgically, as mentioned above. In a simple example, it is common to wear glasses or contact lenses to improve vision by correcting myopic (near-sighted), hyperopic (far-sighted) and astigmatic eyesight. Rather than relying on glasses or contacts, elective laser refractive surgery, or other eye surgery, may serve to improve the refractive state of the eye, including improvement to astigmatism, and may thereby decrease or eliminate dependency on glasses or contact lenses. Such surgeries may include various methods of surgical remodeling of the cornea, or cataract surgery, for example. Surgery may also serve to implant an intraocular lens (IOL), either in addition to the crystalline lens, which addition is referred to as a phakic IOL, or upon removal of the crystalline lens, which replacement is referred to as a pseudophakic IOL.
In particular, an IOL may be a lens implanted in the eye, such as to replace the existing crystalline lens when the crystalline lens has been clouded over by a cataract, for example, or as a refractive element to change the eye's optical power. An IOL is one type of corrective lens that may change the focus of the elements of the eye. This change in focus provided by a corrective lens is herein referred to as defocus.
An IOL may consist of a small plastic lens with plastic side struts, called haptics, to hold the lens in place within the capsular bag. An IOL may be made of a relatively inflexible material, such as polymethyl methacrylate (PMMA), for example, or of a flexible material, such as silicone, acrylic, hydrogels, and the like. An IOL may be a fixed monofocal lens matched to distance vision, for example. An IOL may also be multifocal to provide the recipient with multiple-focused vision at far and reading distances, for example. An IOL may be used to provide the patient with limited visual accommodation, for example.
An IOL may be either spheric or toric. Spheric IOs are used for correction of a myriad of vision problems, while toric IOs are typically used for astigmatic eye correction. When using a toric IOL, the angular orientation of the IOL in the eye is particularly important, as a toric IOL is intended for positioning after insertion at a specific angle, and, in currently available methods, that insertion angle must be maintained, post-insertion, in order to provide a proper astigmatic correction. If the insertion angle is not correct and/or maintained, the astigmatism will not be fully corrected, and in fact the astigmatic condition may worsen. The condition caused by this misalignment of the IOL is often referred to as residual cylinder, or remaining astigmatism.
Generally, astigmatism is an optical defect in which vision is blurred due to the ocular inability to focus a point object into a sharply focused image on the retina. This may be due to an irregular, or toric, curvature of the cornea and/or lens. The refractive error of the astigmatic eye stems from a difference in degree of curvature, and therefore in degree of refraction, of the different meridians of the cornea and/or the crystalline lens, which causes the eye to have two focal points, one correspondent to each meridian. As used herein, a meridian includes one of two axes that subtend a curved surface, such as the prime meridian on the earth, for example. Meridians may be orthogonal. By way of example, the meridians of the earth may be any orthogonal line of longitude and any line of latitude that curve about the surface of the earth.
For example, in an astigmatic eye, an image may be clearly focused on the retina in the horizontal (sagittal) plane, but may be focused behind the retina in the vertical (tangential) plane. In the case where the astigmatism results only from the cornea, the two astigmatism meridians may be the two axes of the cornea. If the astigmatism results from the crystalline lens, the two astigmatism meridians may be the two axes of the crystalline lens. If the astigmatism results from a combination of the cornea and the crystalline lens, the two astigmatism meridians may be the respective axes of the combined lenses of the cornea and the crystalline lens.
Astigmatism arising from the cornea or crystalline lens, or the combination of the two lenses, may be corrected by a toric lens, such as the aforementioned toric IOL. A toric surface resembles a section of the surface of a football, for which there are two regular radii of curvature, one smaller than another. These radii may be used to correct the defocus in the two meridians of the astigmatic eye. Thus, blurred vision caused by astigmatism may be corrected by corrective lenses or laser vision correction, such as glasses, hard contact lenses, contact lenses, and/or an IOL, that provide a compensating optic specifically rotated around the optical axis. However, any misalignment of the compensating optic, and/or improper selection of the corrective lens, may cause residual cylinder, or further astigmatism, and potentially induce other aberrations. The aberrations may be exacerbated if, for example multifocal and toric corrective lenses are required to correct the initial condition, and the respective corrective lenses are misaligned. Similarly, an initial condition may be exacerbated with misalignment of aspheric surfaces used to correct spherical aberration, for example.
Thus, two specific issues arise from using a lens, such as an IOL, to correct astigmatism. First, toric ophthalmic lenses are sensitive to cylinder orientation misalignment relative to that to be corrected, such as wherein the axis of the toric lens in the eye and the lens for correction are not accurately aligned. Second, the cylinder power of the eye or cornea may not sufficiently match the power of the toric IOL. This may be due to measurement errors, unintended changes of cylinder power and/or axis during or after surgery, or because current toric lenses are offered only in a number of discrete cylinder increments.
A need therefore exists for a lens, such as an IOL, having decreased sensitivity to alignment errors and also having decreased sensitivity to selection of the proper cylinder power, and for an optical system and method of providing and using the same.